I should increase F under wat circumstances?
Bigger aperture (smaller f-value) = thinner DOF (this mean less area in a scene are in focus)
Smaller aperture (bigger f-value) = deeper DOF (this mean more area in a scene are in focus).
DOF = depth of field (which mean the distance between the nearest and farthest objects in your picture to remain focused.)
So you might want to understand the distance between yourself and your subject and taking that into your calculation for the aperture. For me... I don't actually do the physical calculation (too much hassle), I trial and error a number of times to get a aperture value that is more or less what I want for a given subject and the distance between myself to the subject.
So in one word - practice more.
Also note that if you are using a smaller aperture, less light will be allowed through your lens to the surface of your sensor, so more time is needed for the shutter to remain open so that the picture is properly exposed, or the sensitivity of the sensor to light needed to be increase (increase in ISO)... for the former, if you have a steady hand and can hold the camera long enough without shivering, shaking and stuff and your subject could hold still for that given amount of time your shutter are open, then you can get a sharp and crisp and almost noise free picture (ISO preferably below 800)... the latter, would definitely increase your shutter speed, but you will get pretty noisy picture, because your ISO is too high (as was demonstrated by your photos).
So one way is to use a tripod, nail your subject to the ground and make sure he or she don't move at all... or use a flash... that would definitely increase your shutter speed... but that would also bring you to another learning level (flash photography).